The puzzle is most frequently a 9x9 grid made up of 3x3 subgrids (called "regions").
Some cells already contain numbers, known as "givens". The goal is to fill in the empty cells,
one number in each, so that each column, row, and region contains the numbers 1-9 exactly once.
Each number in the solution therefore occurs only once in each of three "directions", hence the
"single numbers" implied by the puzzle's name.
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